Organic EL (electroluminescent) displays are conventionally known as being thin display devices featuring high image quality and low power consumption. The organic EL display has a plurality of pixel circuits arranged in a matrix, each circuit including an organic EL element, which is a light-emitting display element driven by a current, and a drive transistor for driving the element.
The method for controlling the amount of current to be applied to current-driven display elements such as organic EL elements as above are generally classified into: a constant-current control mode (or a current-programmed drive mode) in which the current that is to be applied to display elements is controlled by data signal currents flowing through data signal line electrodes of the display elements; and a constant-voltage control mode (or a voltage-programmed drive mode) in which the current that is to be applied to display elements is controlled by voltages corresponding to data signal voltages. Among these modes, when the constant-voltage control mode is used for display on an organic EL display, it is necessary to compensate for variations in threshold voltage among drive transistors and current reduction in the organic EL elements (luminance decay). On the other hand, an the case of the constant-current control mode, the values for data signal currents are controlled such that constant currents are applied to organic EL elements regardless of the threshold voltages and internal resistance of the organic EL elements, and therefore, the compensation as mentioned above is normally unnecessary. However, the constant-current control mode is known to require more drive transistors and more wiring lines than the constant-voltage control mode, which leads to a lower aperture ratio, and therefore, the constant-voltage control mode is widely employed.
In such a configuration employing the constant-voltage control mode, the current to be applied to the organic EL element is determined by the drive (control) transistor, but the potential of the power supply is not always kept constant, and might experience a voltage drop (i.e., an IR drop) due to the resistance of a power supply line and the current flowing through the line.
Particularly in the case of an image where the average tone value of the pixels to be displayed is high (a bright image), the current flowing through the power supply line increases, and therefore, the control voltage of the drive transistor is affected by the aforementioned voltage drop, resulting in a further drop in the voltage, leading to a reduction in display quality, such as deviations in the colors of the image to be displayed or portions with low tones being darkened.
Therefore, to compensate for such a voltage drop, for example, Japanese Laid-Open Patent Publication No. 2004-101767 discloses a display device that is configured to measure currents flowing to organic EL elements and appropriately correct the values of tone voltages to be provided to the drive transistors.
Furthermore, for example, Japanese Laid-Open Patent Publication No, 2010-181877 discloses a display device in which, in addition to a first power supply line, which is a regular power supply line, a second power supply line for voltage drop compensation is provided, and the first and second power supply lines are connected appropriately.